The nervous system comprises billions of neurons, organized into structural and functional assemblages, that perform a wide range of functions For example, some neurons relay information from the central nervous system (CNS) to other parts of the body, while others collect information from peripheral sensors either for use by reflexive systems or for interpretation by the CNS.
One type of organized structure of the nervous system is that of nerves. Nerves are bundles of axons, and may include additional support cells, such as glial cells. A single nerve can contain thousands to upwards of a million individual axons, each axon being a specialized structural modification of a neuronal cell. The body comprises a number of nerves, each one typically serving particular functions or relaying particular types of information to and from particular parts of the body. In general, the relay of information through the nervous system is carried out by the activity of excitable cells such as neurons. Neurons are characterized by the ability to respond to stimuli, to conduct impulses, and to communicate with each other or with other types of responsive cells.
In neurons, this ability arises due to structural and biochemical specializations, the most important of which is the ability to maintain an electrical potential across the cellular membrane of the neuron. This membrane potential is due to the action of integral membrane ion “pumps” that produce and maintain an asymmetric distribution of sodium and potassium ions across the membrane, in which 3 sodium ions are pumped out of the cell in exchange for 2 potassium ions pumped inwards. The net effect in a typical neuron is that the electric potential difference across the membrane is typically in the range of −70 mV, referred to as a resting potential.
In addition to mechanisms that produce the asymmetric distribution of sodium and potassium ions that create the resting potential, excitable cells like neurons also have structural and biochemical mechanisms that result in depolarization of the cell membrane, resulting in a wave of electrical activity that propagates along the surface of the neuron.
Depolarization can be caused either by chemical or direct electrical stimulation of the cell. Typically depolarization occurs initially as a localized event on the neuron cell surface that results in the opening of voltage-gated sodium channels. Opening of these channels allows sodium to diffuse into the cell driven along its electrochemical gradient. This results in a reduction in the potential difference across the membrane, which in turn opens more voltage-gated sodium channels, allowing more sodium into the cell, which further depolarizes the cell. Once a threshold level is reached, the cells will completely depolarize, leading to the production of an action potential.
Once initiated, an action potential will be propagated down the length of the neuron, for example, down the axonal portion of the cell. The speed of conduction is dependent on the diameter of the axon, as well as other factors, such as whether the nerve is myelinated or non-myelinated. Larger diameter neurons generally conduct action potentials more rapidly, as do fibers that are myelinated. Stimulation of neuronal signaling can occur naturally in a number of ways. For example, some neurons have cell surface receptors that bind to specific signaling molecules. In response to ligand binding, the receptors in turn signal ion channels to open or close, which can lead to depolarization or hyperpolarization of the neuron. Hyperpolarization leads to de-sensitization of the nerve, while depolarization sensitizes the nerve and increases the likelihood that a stimulus will result in the production of an action potential.
Neurons can be artificially stimulated to depolarize by application of an electrical signal. In these cases, the electrical signal directly acts on voltage-gated channels in the cell membrane leading to depolarization. If a signal of sufficient intensity is applied, an action potential can be evoked.
In addition to the production of an electrical signal, neurons in particular are also able to provide information coding, depending on the frequency at which depolarization occurs, the timing of depolarization or even simply on whether the neuron is firing or not.
Cuff electrodes are well known in the neurostimulation field. Cuff electrodes can be used to stimulate and/or measure the response of peripheral nerves. A cuff electrode can wrap around the nerve to be stimulated and/or sensed.
At least one prior art nerve cuff includes a sheet biased to curl into a tubular spiral when released or wrapped around a nerve. Applicants believe this design is less than optimal for at least two reasons. The first reason is manufacturability. A nerve cuff may have nominal dimensions of 1 cm by 1 cm. One method for biasing the sheet to curl is to stretch a first sheet and adhere the first stretched sheet to a second sheet, allowing them to securely bond. Electrodes would presumably be secured to the first sheet or within recesses in the first sheet. Applicants are unsure as to the reproducibility of such a process with respect to the inwardly directed force on a nerve, among other properties.
A nerve may be teased out of the surrounding tissue, often using blunt dissection tools. This isolation of the nerve may irritate the nerve, which may lead to swelling of the nerve, as would be expected with many other tissues. This swelling can increase the outer diameter of the nerve.
During placement of the nerve cuff, the inwardly directed pressure of the cuff should fall between two extremes, both disadvantageous. If the cuff applies too much pressure on the nerve, the nerve can be damaged. If the cuff applies an initially proper amount of pressure and the nerve swells, then too much pressure may be applied if the cuff does not expand enough.
If the cuff does not apply enough pressure on the nerve, this often means that the cuff is not closely fitted to the nerve, and the cuff can become dislodged from the nerve, particularly during placement. This can allow an undesirable amount of fibrotic tissue ingrowth. This can also force the current applied to the cuff electrode to be larger than optimal, shortening battery life and perhaps even allowing stray currents to effect nearby tissue. If the cuff is initially properly situated, and the nerve later returns to normal size, then the cuff should shrink in order to maintain the proper fit around the nerve.
What would be advantageous is a nerve cuff which can be easily placed using minimally invasive techniques. What would be beneficial is a nerve cuff which is self sizing yet efficient and has a well defined closing force.